A very good description of the prior art appears in the textbook, "K. Szechy, Tunnelbau"--1969, Springer Verlag, Vienna--New York, pages 686 through 697 (in German).
It is a specific disadvantage of the prior art that if it is desired to lay out a tunnel, which must follow a predetermined spatial curve, which may include uphill and downhill portions, or portions going to the right or the left, a special matching section has to be provided, which requires mating with receiving and following sections of the tunnel. This has been achieved primarily by employing conically-truncated rings, as matching pieces. In view of unavoidable constructual deviations from those planned during the forward motion of the propelling shield, the number and the placement of the conically-truncated rings could not be planned exactly in advance. The combination of parallel cylindrical rings with truncated cylinders or cones required a correspondingly large number of pipe sections and conduits forms, and in turn resulted in a large storage of all types of pipes, including associated constructual parts.
Both the prior art systems and the system according to the present invention, require means to prevent surface subsidence.
Methods to prevent such subsidence are discussed in "Tunnelling Technology", an appraisal of the state of the art for application to transit systems, published by the Ministry of Transportation and Communications of Canada, in 1976. Of particular interest is Section 6, pages 96 and 97, the applicable portions of which are briefly summarized below.
One method of preventing surface subsidence is grouting or caulking. When proper workmanship is carried out, the effect of potential ground subsidence can be minimized.
Backfilling and grouting operations can have several functions including: (i) stabilization of the surrounding ground, thus reducing possible ground-pressure on the lining; (ii) sealing of the ground against water; and (iii) establishment of a tight backfill to minimize distortion and subsidence.
These functions are not normally carried out by the same construction operation. Primary grouting is intended to fill the void space and to develop a tight backfill. It is normally carried out under low pressures of 70 to 80 pounds per square inch. Secondary grouting is used to stabilize the ground and reduce seepage and usually requires a pressure of at least 160 pounds per square inch.
For primary grouting, a fairly wet cement grout can be used by injecting through grout pluds in the lining as the tail shield clears them. However, this system has disadvantages in that large quantities of cement are required, and if the grout is too liquid, it will flow around the segments and interfere with the shield. The quantity of cement can be reduced by the use of pea gravel which is blown into the annular space first before the cement grout. This is done by means of special equipment which operates under the same air pressures required for the cement grouting.
Depending upon the stand-up time, the primary grouting may have to be carried out shortly after the shield has advanced. Under adverse conditions, the pea gravel may have to be introduced immediately to minimize the potential ground movement and thus an additional construction operation must be included at the tunnel face and within the critical path. Unless the operation is carefully controlled, the rate of advance will decrease and an increase in the cost of construction will result. If there is sufficient stand-up time, the grouting may be delayed until the maintenance period for servicing the excavating equipment.
Secondary grouting for sealing and stabilization should either be carried out in advance of the excavation process, or several months after primary grouting, when the ground conditions have achieved equilibrium. In the latter case, the grouting pressures must be carefully checked for the development of excess pressures and non-uniform loading on the completed tunnel lining. The highest pressures that can be used will be related to the depth of overburden and the permissible deformations of the lining. In many cases, these limiting pressures will be below those necessary to force the grout into very fine soils.
To control seepage into the tunnel, it is necessary to incorporate caulking grooves in the tunnel segments, and to fill the spaces with caulking compounds of asbestos cement, hemp or lead. With carefully constructed lining segments, the amount of seepage is relatively minor so that the caulking operations can usually be carried out at a convenient later stage in the construction program.
Another method of preventing surface subsidence is the use of extruded and slipform tunnel lining systems. Slipforming is applicable to both rock and soft ground tunnelling conditions, although the method of operating the tunnel boring machine (TBM) equipment and the jacking sequences would have to be different for each of these main ground classifications.
The ideal material for use in forming an extruded liner should possess the following properties: It should (i) be chemically stable and non-toxic; (ii) be pumpable; (iii) solidify rapidly with little volume change under controlled rates of setting; (iv) exhibit a rapid gain in strength; (v) gain strength with time; (vi) be dimensionally stable; (vii) be corrosion resistant; and (viii) be low in overall economic cost.